This invention relates to an electroconductive integrated substrate and a process for producing the same which is suitable for a functional element of an electrode substrate unit, fuel or oxidative gas passage unit, cooling unit and the like for a fuel cell containing acid or alkali as an electrolyte, particularly a phosphoric acid fuel cell, or a membrane fuel cell using an ion-exchange membrane as an electrolyte, and a functional element of an electrode substrate unit etc. for secondary batteries of electrolyte circulation type.
A fuel cell is, as is known in the art, structured by stacking a plurality of electrode substrate units each comprising a gas separating plate with electroconductivity sandwiched between gas permeable electrode substrates with electroconductivity. In such a unit, if the electrical contact resistance at the surface between the gas separating plate and the electrode substrate is large, the voltage drop is naturally great when a current is passed in the stacking direction and, in the case of fuel cell, the electric efficiency is low.
To overcome this problem, an electrode substrate unit has been proposed, for example, in Japanese Laid-Open Patent Publication No. 61-19069, wherein porous electrode substrates with gas permeability, made of carbon fibers, are bonded to both surfaces of a carbonaceous gas separating plate with electroconductivity by a phenolic resin and then the resulting structure is heat treated to carbonize the resin. Since the resin is carbonized, this unit has a lower electrical contact resistance as compared to a structure in which the gas separating plate and the porous electrode substrates are merely laminated. There is, however, the problem that the manufacturing cost is high because of the necessity of the heat treatment step. Moreover, since the resin shrinks during the heat treatment step, the unit may be warped or fail to provide sufficient bonding strength, causing the problem of the bonded surfaces being separated. Naturally, these inconveniences are more prominent in the case of larger-sized area unit.
In Japanese Laid-Open Patent Publication No. 62-211868 is disclosed an electroconductive gas separating plate which is fabricated by sandwiching a thermoplastic resin film between two porous carbonaceous plates with gas permeability and bonding these together with application of heat and pressure to form an integral structure. As the thermoplastic resin film, the Japanese Publication discloses using a polytetrafluoroethylene (PTFE) film. However, because of its melting viscosity as high as 10.sup.10 -10.sup.13 poises, PTFE, if melted with application of heat, does not penetrate satisfactorily into the pores of the gas permeable porous carbonaceous plate and thus fails to provide sufficient bonding strength. Furthermore, PTFE remains in the form of a thick layer at the bonded surfaces. Therefore, the electrical resistance at the bonded surfaces of adjacent gas permeable porous carbonaceous plates are large.
Japanese Laid-Open Patent Publication No. 63-48763 discloses an electrode substrate for a fuel cell, which is produced by bonding a porous carbonaceous plate and a separator by a "Teflon" dispersion. The "Teflon" dispersion is prepared by dispersing 10-70% by weight of polytetrafluoroethylene (PTFE), and after this dispersion is applied to the bonding surface of the separator, the porous carbonaceous plate is attached to the bonding surface of the separator and the two are bonded together with application of heat and pressure. The electrode substrate obtained in this manner is, however, poor in electroconductivity due to the use of PTFE dispersion, and it is difficult to apply PTFE uniformly to the bonding surface, thus failing to provide a desired bonding strength.